The present invention relates to a filtering system, and more particularly to a scheme for low-pass filtering an amplitude limited signal without overshooting the predetermined amplitude constraints thereof. The system is specifically described with reference to FM stereo broadcasting.
In systems requiring both peak amplitude limiting and bandwidth limiting of a signal, problems arise in jointly meeting both of these requirements due to the tendency of a circuit providing either of these limitations to adversely effect the other limitation. Thus, a bandwidth limiting filter generally has a tendency to cause peak amplitude overshoots, whereas a peak amplitude limiting circuit has a tendency to produce spurious signals which will generally fall outside the bandwidth constraints. Because of this, regardless of which of the limitations is initially applied to the signal, in achieving the second limitation, the first will be lost.
This is particularly critical in FM stereo broadcasting, where the FCC has placed both bandwidth and modulation constraints on the signal to be broadcast. These requirements have generally been met in the past by reducing the amplitude level of the modulating signal to the point at which the overshoots caused by subsequent bandwidth limiting filters will not produce over-modulation of the broadcast signal. This can mean a sizeable reduction in modulation effectiveness, usually on the order of 2 1/2 to 6 db.
It would thus be desirable to provide a system for frequency limiting an amplitude limited audio signal in such a manner that the average modulating level need not be reduced to prevent overshoot. An optimal system for performing this function must have a flat frequency response in the passband of the filter at all amplitude levels up to 100% modulation, must have a high attenuation of frequencies outside of the passband, must reduce over-modulation due to filter overshoot to an insignificant level, must not degrade the quality of the signal being processed, and must have insignificant THD and IM distortion at any modulating level of the 100%.
The present invention provides a new filtering technique that eliminates over-modulation due to amplitude overshoot while still meeting all of the above requirements.
In accordance with the present invention, apparatus is provided for filtering an amplitude limited signal. A first filter is responsive to the amplitude limited signal to produce a filtered signal which may include an amplitude overshoot component due to the filtering. An overshoot compensator is provided which is responsive to this filtered signal and which modifies the signal so as to provide a modified filtered signal which, when filtered for a second time, will produce a frequency limited signal which does not exceed predetermined amplitude constraints.
In accordance with the more limited aspect of the present invention, overshoot compensator means of a particular type is set forth. The overshoot compensator includes a threshold clipper which is responsive to the filtered signal for separating out the overshoot component from the filtered signal. Means are provided for increasing the amplitude of the overshoot component which has been separated from the filtered signal and for then subtracting this amplified component from the filtered signal so as to provide the modified signal.